The deposition of paraffin on tubing and flow lines is a persistent problem in oil production. Paraffin deposition causes restricted flow of the fluids being produced from the well, increased flow line pressure, and decreased production, and can lead to mechanical problems. Paraffin is generally removed from tubing by mechanical, thermal, or chemical means, each method having various advantages and disadvantages. Mechanical methods rely on the scraping of the paraffin from the surface of the pipe and are, therefore, generally used when the paraffin deposits are not very heavy. Large deposits of paraffin physically obstruct the pipe and prevent the passage of the scraper such that mechanical methods are unsatisfactory for use in that situation.
Chemical methods of treating paraffin include the circulation of various solvents, wax crystal modifiers, and/or paraffin dispersants down the production tubing. However, the paraffin is then carried in the tubing with the fluid all the way to the refinery, and some chemicals are not desirable at the refinery. Further, treatment of the well with chemicals is costly compared to mechanical and thermal treatment, mostly because paraffin removing chemicals are generally fairly expensive.
Thermal methods involve the pumping of hot oil down the well outside of the production tubing and using heat transfer through the production tubing to melt the paraffin on the inside of tubing. After the paraffin inside the tubing is melted, hot oil must be pumped down the flow line to clean the paraffin out of the flow line. However, pumping oil down the outside of the production string and down the inside of the flow line requires a greater quantity of oil and is thus more expensive. Further, heat transfer is not always effective in removing all of the paraffin. Other disadvantages of chemical and thermal methods of paraffin removal are addressed in the specification of U.S. Pat. No. 4,681,167, and that discussion is hereby incorporated into this specification by this specific reference thereto.
There is, therefore, a need for an improved method for removing paraffin deposits from a well, a problem which has been recognized in several previously issued U.S. patents. For instance, U.S. Pat. No. 3,085,629 discloses a control coupling to be inserted into a production string, or tubing, of a type commonly used in the industry just below the point at which paraffin accumulates. The coupling includes a valve having an opening through the wall thereof which is spring-biased to a normally closed position as oil is produced from the well. Introduction of liquid at high pressure, however, opens the spring loaded valve, allowing the passage of the pressurized fluid therethrough once the tubing string has been filled with fluid to produce a sufficient pressure to overcome the tension of the spring.
As described in that patent, downward flow through the production tubing is prevented by a one way valve that is part of the pump located at the bottom of the well and which prohibits fluids in the tubing from flowing down into the strata. As noted in the above-referenced U.S. Pat. No. 4,681,167, such an apparatus does not provide for the unloading, e.g., the draining, of the pressurized fluid which has been introduced into the tubing string to remove the paraffin from the tubing string. Unloading must be accomplished if, for instance, the pump is stuck in the well. Draining a well in which an apparatus such as that disclosed in U.S. Pat. No. 3,085,629 is positioned requires the backing off of the sucker rods off the installation at the pump, pulling the rods, and then swabbing the well to remove the fluid or shooting a hole in the tubing to drain the fluid. Either method is expensive and time consuming.
U.S. Pat. No. 3,085,629 is merely representative of several prior patents that disclose devices which are utilized in a producing well having production installations of a type commonly used in the industry which include sucker rods, a downhole pump, and a tubing string which is closed by a one-way valve in the pump at the bottom of the well but which do not allow the draining of fluid from the tubing. Those patents of which Applicant is aware that describe devices that are characterized by that same limitation are identified by the following numbers: U.S. Pat. Nos. 3,376,936, 4,330,039, 3,542,130, 4,434,854, 4,049,057, 4,645,007, 4,257,484, 4,681,167.
Other disadvantages and limitations of several of the devices disclosed in these patents are characterized in the specification of U.S. Pat. No. 4,357,484, and that discussion is also hereby incorporated into this specification by this specific reference thereto.
In short, and as set out in the specification of U.S. Pat. No. 4,681,167, there remains a long-felt, but unfulfilled, need within the industry for an apparatus which can be selectively shifted between a closed position for producing fluids from a well and an open position for introducing a fluid into a producing well which can also be maintained in the open position to permit draining of the production tubing. The apparatus described in that particular patent has not, in spite of the stated intention of doing so, fulfilled that need. That patent describes a piston valve for positioning in a production installation of the type described above which is shifted to an open position for introducing fluid into the well by increased pressure in the production tubing. An engaging device mounted on the sucker rods which extend down through the valve is provided for subsequently closing the valve by mechanical action. Closing the valve is accomplished from the surface by a pivoting dog mounted on the piston which is selectively pivoted to engage the engaging device. That pivoting dog bears against the outside surface of the engaging device until actuated. Of course, the sucker rods of the production installation continually reciprocate within the valve, and on information and belief, the pivoting dog eventually wears through the engaging device as a result of the constant reciprocation, defeating the alleged reliability of the valve. Further, the device must be manually shifted to the closed position, e.g., does not close when the pressure of the fluid in the tubing is reduced.
The use of fluid pressure changes to open and close a valve is, of course, known in the industry. For instance, U.S. Pat. No. 3,627,049 discloses a device having normally closed, pressure-actuated valves arranged at opposite ends of the tubing string which are adapted to be successively opened. However, that apparatus is adapted for removing contaminates that have previously entered the producing formation following perforation of the casing by sudden change in pressure rather than for circulation of paraffin-removing fluid, and is not conveniently adaptable for removing paraffin from a well. U.S. Pat. No. 4,574,894 discloses a pressure-activated dump valve for use in a downhole motor. Although the device opens and closes in response to the pressure of the fluid in the tubing in which it is located, operation of that device is in a manner opposite the operation required for circulating paraffin-removing fluids in that pressure is required to close rather than to open a fluid flow port.
It can be seen, therefore, that the need for a pressure-operated downhole valve for circulating fluids down into the well for paraffin removal which also allows the draining of fluid from the tubing string in which the valve is located that was recognized in U.S. Pat. No. 4,681,167 still has not been fulfilled. The present invention, however, provides such a valve.
It is, therefore, an object of the present invention to provide a pressure-operated valve for use in a well having a production installation of a type in common use in the industry for circulating fluids through a tubing string for the removal of paraffin from a well and for draining fluids from the well.
It is another object of the present invention to provide a circulating valve which is effective in removing paraffin from a well under a variety of well conditions.
It is another object of the present invention to decrease the cost of removing of paraffin from a well.
Still another object of the present invention is to provide a circulating valve having ports through the wall thereof of different cross-sectional dimensions for maintaining the valve in the open, circulating position as long as the pressure of the fluid in the tubing is maintained above a selected pressure.
It is another object of the present invention to provide a combination circulating valve and tubing unloader, the valve of which does not close in response to pressure decreases, when the pressure of the fluid in the tubing string in which the valve is located exceeds a selected pressure.
It is another object of the present invention to provide a valve which, once opened for draining of fluid from the tubing, remains open regardless of the pressure of the fluid in the tubing.
Other objects, and the advantages of the present invention, will be apparent from the following description of a presently preferred embodiment thereof.